This invention relates to the fabrication of composite structures and more particularly to the fabrication of high strength-low weight laminated structures of substantially unlimited size without the use of high cost autoclaves by the application of internal pressure to the structure while providing an external pressure restraint on the structure.
It has been demonstrated that significant weight and cost savings can be attained over aluminum and steel alloys in structures incorporating advanced composite materials. Fifteen to forty percent savings have been generated in various aircraft and aerospace components. Five to twenty five percent cost savings have been recorded in production programs. These benefits are attributed to high strength, high stiffness fibers in a low density matrix, both metallic and non-metallic, which can be oriented to address maximum load conditions. For example, high strength composite structures having graphite fibers embedded in an epoxy matrix have approximately nine times the specific strength of stainless steel and three and one-half times that of aluminum alloys, and have specific moduli which are approximately 22 times and 3.2 times that of stainless steel and aluminum alloys respectively. High modulus types of graphite/epoxy composites are in the order of seven and one-half and two and one-half times the specific strength of stainless steel and aluminum alloys respectively, and have a specific modulus which is in the order of approximately thirty and four and one-half times that of stainless steel and aluminum alloys respectively. A composite made up of fibers of a polyaramide sold under the registered trademark KEVLAR and an epoxy matrix has a specific strength of approximately eleven times that of stainless steel and 3.75 times that of aluminum, while the specific modulus is in the order of approximately fifteen and two times that of these respective metals.
Difficulties in processing the matrix material has been experienced due to the required application of temperature and pressure. The processing or fabrication of these materials is typically conducted in an autoclave allowing the simultaneous application of pressure and temperature. Autoclaves, unfortunately, are extremely costly to build, and only small autoclaves have been constructed. An autoclave of sufficient size to fabricate a typical boost vehicle of approximately 30 feet in diameter has never been made because the cost would be prohibitive. In practice, the largest aerospace composite structural component is the 14 foot diameter graphite/epoxy solid rocket motor case manufactured by Hercules Aerospace, Inc. Fabrication of this structure was conducted in an oven with pressure being applied using shrink tape and a vacuum (14 psi). The resulting structure was porous and of a significantly lower strength, in the order of approximately fifteen to thirty percent strength reduction, than that estimated to be attainable with the application of greater pressures in the order of approximately 85 psi.
Studies conducted by the U.S. Air Force of graphite/epoxy composites process in autoclaves under 85 psi pressure versus non-autoclave cure cycles under pressures of approximately 15 psi typically show a fifteen to forty five percent strength reduction and porosity/delaminations in structures. Additional material is usually added to improve the situation by increasing the resin content to allow better flow, which reduces the strength-to-density ratio, and by adding more material to achieve required performance. Both of these solutions, however, increase both the cost and the weight of these structures.
Additionally, when using autoclave curing of composite materials when the pressure is applied to thick laminate disposed around the mandrel, the external pressure reduces the bulk factor and makes the composite structure thinner and more dense. The outer plies, having a fixed circumference, are suddenly too long and distort or buckle as they are compressed and forced to reposition into a smaller volume. Such distortion results in flaws which are detrimental to the structure.
Recent studies on boost vehicles, e.g., the NSTS External Tank, the Advanced Launch System and the Titan II, indicate significant weight savings may be achieved by using resin-matrix advanced composite such as graphite/epoxy for the intertank structure. However, the technology of fabricating a structure of this size has yet to be developed. In addition, the design, fabrication and cost of an autoclave required to ensure producability for a filament wound or cocured structure would make non-recurring production costs prohibitively high.
Metal-matrix composite fabrication presents further complications. In general, two types of starting material are available to make either monolayer tape or multilayer sheet, plate and structural shapes, these being filaments and metal or a previously prepared tape. There are four basic forms of tape (1) filaments bonded to foil sheets with a polymeric binder, e.g., acrylic or polystyrene, known as green tape; (2) filaments bonded to foil by an overlay of plasma sprayed matrix metal, known as plasma sprayed tape; (3) filaments sandwiched between two sheets of metal that have been diffusion bonded together, known as diffusion bonded tape; and (4) reinforced filaments and matrix metal wire woven together, known as woven tapes. Sheet, plate or structural shapes are then formed by these materials by diffusion bonding, braze bonding or eutectic bonding. Although low pressure processes of less than 200 psi have been pioneered using braze bonding and eutectic bonding, material properties are not comparable to those obtained by high pressure diffusion bonding. High pressure diffusion bonding is the most developed process having been applied to various space shuttle structures. This process is performed in an autoclave or in a press at pressures between 3000 and 10,000 psi, and temperatures of 850.degree. F. to 1000.degree. F. Hot presses are generally limited to the production of flat panels either having constant or varying cross section, and autoclaves are required for fabrication of structural shapes such as tubes, hats, tees, and similar shaped structures. Thus, lack of fabrication ease limits the wide spread application of these metalmatrix materials and their benefits have not been realized to the full extent.